Precipitated iron catalyst for hydrogenation of carbon monoxide

ABSTRACT

A method of producing an iron catalyst for catalyzing the hydrogenation of carbon monoxide is disclosed. The method comprises using a reduced amount of acid for iron dissolution compared to certain previous methods. The resulting acidic iron mixture is heated without boiling to obtain a nitrate solution having a Fe 2+ :Fe 3+  ratio in the range of about 0.01%:99.99% to about 100%:0% (wt:wt). Iron phases are precipitated at a lower temperature compared to certain previous methods. The recovered catalyst precursor is dried and sized to form particles having a size distribution between 10 microns and 100 microns. In embodiments, the Fe 2+ :Fe 3+  ratio in the nitric acid solution may be in the range of from about 3%:97% to about 30%:70% (wt:wt) and the calcined catalyst may comprise a maghemite:hematite ratio of about 1%:99% to about 70%:30%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application under 35 U.S.C. §121 ofU.S. patent application Ser. No. 12/189,424, now U.S. Pat. No.7,879,756, filed Aug. 11, 2008, which claims the benefit under 35 U.S.C.§119(e) of U.S. Provisional Patent Application No. 60/955,142 filed Aug.10, 2007 and U.S. Provisional Patent Application No. 61/022,566 filedJan. 22, 2008, the disclosures of each of which are hereby incorporatedherein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

1. Technical Field

The present invention generally relates to processes for hydrogenatingcarbon monoxide, and more particularly relates to iron-basedcompositions for catalyzing such processes, and still more particularlyto the manufacture of promoted iron-based catalysts.

2. Description of Related Art

The Fischer-Tropsch process is a well known catalyzed reaction in whichcarbon monoxide and hydrogen are converted into various forms ofhydrocarbons. Catalysts for the reaction are commonly based on iron,sometimes using a precipitated iron-based catalyst that also containssome type of promoter to improve catalyst stability or to affect theproperties of the hydrocarbons produced.

U.S. Pat. No. 5,504,118 describes Fischer-Tropsch reaction schemes usingcertain iron catalysts promoted with potassium and copper in a slurryreactor to produce hydrocarbon products having more than five carbonatoms, water, and alcohols.

German Patent No. 763864 describes certain methods of making ironcatalysts for production of hydrocarbons from carbon monoxide andhydrogen under normal or increased pressure (5-50 atm). The catalystscontain bi- and trivalent iron salts and up to 0.5% copper, and are madeby heating and precipitating the solutions.

There is continuing interest in the development of iron-based catalystsfor catalyzing the hydrogenation of carbon monoxide to formhydrocarbons.

BRIEF SUMMARY

In accordance with certain embodiments of the invention, a method ofmaking an iron catalyst is provided which comprises using a reducedamount of acid for dissolution of the iron starting material, comparedto conventional methods. In embodiments, the effective amount of acid inthe dissolution of iron produces a nitrate solution having both ferrous(Fe²⁺) and ferric (Fe³⁺) ions. In embodiments, the presence of ferrousions increases the amount of lepidocrocite (γ-FeOOH) and/or magnetite[iron (II,III) oxide; Fe₃O₄] relative to goethite (α-FeOOH) and/orferrihydrite (Fe₅HO₈.4H₂O) precipitated from the solution. The increasein the amount of magnetite and/or lepidocrocite relative to ferrihydriteand/or goethite leads to an increased maghemite (γ-Fe₂O₃) to hematite(α-Fe₂O₃) ratio in the raw catalyst product. For example, in someembodiments, the presence of ferrous ions increases the amount ofmagnetite relative to ferrihydrite (Fe₅HO₈.4H₂O) precipitated from thesolution which in turn leads to an increased maghemite (γ-Fe₂O₃) tohematite (α-Fe₂O₃) ratio in the raw catalyst product.

In certain embodiments, a method of manufacturing a catalyst comprisingiron, copper and potassium is provided which comprises: preparing aniron nitrate solution having a Fe²⁺:Fe³⁺ ratio (wt %/wt %) in the rangeof from about 0.01% Fe²⁺:99.99% Fe³⁺ to about 100% Fe²⁺:0% Fe³⁺ andoptionally comprising copper; heating at least a portion of the ironnitrate solution to a temperature in the range of about 20° C. to 80°C.; preparing a precipitating agent solution; reducing the temperaturesof the iron nitrate solution and the precipitation agent solution torespective temperatures in the range of 25° C. to 35° C., to obtainrespective low temperature solutions; and reacting the low temperaturenitrate solution with the low temperature precipitating agent at atemperature not exceeding 40° C., to form a precipitate comprising Fe²⁺and Fe³⁺ phases optionally copper phases. The phases may comprisehydroxides, carbonates, oxides, or any combination thereof. The methodmay further comprise: ripening the precipitate; washing the resultingripened precipitate to remove nitrates; aging the resulting washedprecipitate; slurrying the resulting aged precipitate and adding achemical promoter; drying the resulting slurry to form a catalystprecursor; calcining the catalyst precursor to form a raw catalyst; orany combination of at least one of these.

In some embodiments, the method further comprises activating the rawcatalyst by exposure to a gas comprising carbon monoxide, hydrogen, or acombination thereof for a selected period of time at selected levels ofpressure, temperature, and space velocity sufficient to enhancecatalytic activity for hydrogenating carbon monoxide to form higherhydrocarbons.

In embodiments, the catalyst preparation method comprises preparing aniron nitrate solution having a Fe²⁺:Fe³⁺ ratio (wt %/wt %) in the rangeof from about 3% Fe²⁺:97% Fe³⁺ to about 30% Fe²⁺:70% Fe³⁺ and comprisingcopper. In embodiments, this comprises mixing together a selected amountof metallic iron or an iron-containing compound and a selected amount ofmetallic copper in a selected amount of nitric acid having a specificgravity greater than 1.01 and less than 1.40. In certain embodiments,mixing together a selected amount of metallic iron or an iron-containingcompound and a selected amount of metallic copper in a selected amountof nitric acid having a specific gravity greater than 1.01 and less than1.40 comprises separately preparing a copper nitrate solution and aniron nitrate solution, and combining the separately-prepared solutionsto form the iron nitrate solution comprising copper.

In certain embodiments, preparing the iron nitrate solution having aFe²⁺:Fe³⁺ ratio (wt %/wt %) in the range of from about 0.01% Fe²⁺:99.99%Fe³⁺ to about 100% Fe²⁺:0% Fe³⁺ comprises preparing a ferrous nitratesolution by adding metallic iron or an iron-containing compound and aselect amount of nitric acid having a specific gravity less than 1.035and a ferric nitrate solution by adding metallic iron or aniron-containing compound and a select amount of nitric acid having aspecific gravity greater than 1.115 and combining them to provide aniron nitrate solution having a specific gravity greater than 1.01 andless than 1.40.

In some embodiments, the weight of nitric acid is 2.8 to 4.5 times theweight of iron in the iron nitrate solution. In some embodiments, theweight ratio of copper to iron in the iron nitrate solution comprisingcopper is in the range of 0.002 to 0.02.

In other embodiments, the catalyst preparation method comprises mixingtogether a selected amount of metallic iron or an iron-containingcompound in a selected amount of nitric acid having a specific gravitygreater than 1.01 and less than 1.40, to obtain an iron nitrate solutionhaving a Fe²⁺:Fe³⁺ ratio (wt %/wt %) in the range of about 0.01%Fe²⁺:99.99% Fe³⁺ to 100% Fe²⁺:0% Fe³⁺. In these embodiments, copper isadded as copper nitrate just prior to spray drying. In theseembodiments, preparing the iron nitrate solution having a Fe²⁺:Fe³⁺ratio (wt %/wt %) in the range of from about 0.01% Fe²⁺:99.99% Fe³⁺ toabout 100% Fe²⁺:0% Fe³⁺ may again comprise preparing a ferrous nitratesolution by adding metallic iron or an iron-containing compound and aselect amount of nitric acid having a specific gravity less than 1.035and a ferric nitrate solution by adding metallic iron or aniron-containing compound and a select amount of nitric acid having aspecific gravity greater than 1.115 and combining them to provide aniron nitrate solution having a specific gravity greater than 1.01 andless than 1.40.

In embodiments, an effective ratio of maghemite to hematite is achievedwherein the ratio of percent Fe²⁺ to percent Fe³⁺ in the iron nitratesolution is in the range of from about 3%:97% to about 30:70 (w/w). Insome embodiments, the ratio of Fe²⁺ to Fe³⁺ is about 25%:75% (w/w). Inspecific embodiments, the percentage weight ratio of Fe²⁺ to Fe³⁺ isabout 3.3%:96.7%. In other specific embodiments, the percentage weightratio of Fe²⁺ to Fe³⁺ is about 10%:90%, as in Example 1 hereinbelow. Incertain embodiments, the Fe²⁺ to Fe³⁺ weight ratio in the resultingnitrate solution after the heating is about 30%:70%. Example 2hereinbelow describes an embodiment in which the Fe²⁺ to Fe³⁺ weightratio in the resulting nitrate solution after the heating is 100%:0%.Example 3 describes an embodiment in which the Fe²⁺:Fe³⁺ weight ratio inthe resulting nitrate solution after the heating is 20%:80%. Example 3describes an embodiment in which the Fe²⁺:Fe³⁺ weight ratio in theresulting nitrate solution after the heating is 20%:80%. Example 4describes an embodiment in which the Fe²⁺:Fe³⁺ weight ratio in theresulting nitrate solution after the heating is 50%:50%. Example 5describes an embodiment in which the Fe²⁺:Fe³⁺ weight ratio in theresulting nitrate solution after the heating is 80%:20%. Example 6describes an embodiment in which the Fe²⁺:Fe³⁺ weight ratio in theresulting nitrate solution after the heating is 0%:100%.

The catalyst preparation method comprises heating the iron nitratesolution to a temperature in the range of about 20° C. to 80° C. Inother embodiments, the iron nitrate solution is heated to a temperaturein the range of from about 40° C. to about 80° C. In some specificembodiments, the iron nitrate solution is heated to a temperature ofabout 40° C. In other specific embodiments, the iron nitrate solution isheated to a temperature of about 55° C. In some embodiments, thetemperature of the mixture is maintained in the range of 25° C. to 80°C. In embodiments, the iron nitrate solution is heated at a rate oftemperature increase in the range of from 1° C./min to 20° C./min. Incertain embodiments, the iron nitrate solution is heated to about 70° C.at a rate of about 3° C./min.

The catalyst preparation method comprises preparing a precipitatingagent solution. In some embodiments, the precipitating agent comprises acompound selected from the group consisting of NH₄OH, Na₂CO₃, NaOH,K₂CO₃, KOH, (NH₄)₂CO₃, (NH₄)HCO₃, NaHCO₃ and KHCO₃.

The catalyst preparation method comprises reducing the temperatures ofthe iron nitrate solution to a temperature in the range of from about25° C. to about 35° C. to obtain a low temperature iron nitrate solutionand reducing the temperature of the precipitating agent solution to atemperature in the range of from about 25° C. to about 80° C. to obtaina low temperature precipitating agent solution, and precipitating aprecipitate comprising Fe²⁺ and Fe³⁺ phases and optionally copper phasesby reacting the low temperature nitrate solution with the lowtemperature precipitating agent at a temperature not exceeding 40° C. Inembodiments, the precipitating agent is selected from NH₄OH, NaOH, KOH,and combinations thereof, and the low temperature precipitating agentsolution has a temperature in the range of from about 25° C. to about35° C. In embodiments, the precipitating agent is selected from Na₂CO₃,K₂CO₃, (NH₄)₂CO₃, (NH₄)HCO₃, NaHCO₃, KHCO₃ and combinations thereof, andthe low temperature precipitating agent solution has a temperature inthe range of from about 25° C. to about 35° C.

In certain embodiments, the use of low temperature precipitation, asdescribed above, allows for greater control over pH during theprecipitation procedure and also allows for improved copper retention,an increase in crystallinity and pore size, and a decrease in surfacearea, pore volume, and/or crystallite size.

In some embodiments, the catalyst preparation method further comprisesripening the precipitate. In embodiments, ripening the precipitatecomprises ripening the precipitate for a period of time ranging from 30minutes to 60 minutes. In some embodiments, the catalyst preparationmethod further comprises washing the precipitate. In embodiments, thecatalyst preparation method further comprises washing the resultingripened precipitate. In some embodiments, the catalyst preparationmethod comprises aging the ripened precipitate or the washedprecipitate. Aging may comprises aging for a period of time ranging from10 minutes to 30 days. In some embodiments, aging may comprises agingfor a period of time ranging from 10 minutes to 240 minutes.

In some embodiments, the catalyst preparation method comprises slurryingthe precipitate and adding a chemical promoter. In embodiments, thechemical promoter comprises a potassium compound selected from the groupconsisting of K₂CO₃, KHCO₃, and KOH. The weight ratio of potassium toiron in the slurry may be between 0.5 K:100 Fe and 1.5 K:100 Fe, forexample.

The catalyst method may comprise drying the slurry to form a catalystprecursor. In certain embodiments, the catalyst precursor comprisesparticles having a size distribution of from about 10 microns to about100 microns.

The catalyst preparation method may comprise calcining the catalystprecursor. In certain embodiments, calcining comprises calcining thecatalyst precursor according to the following program: ramping thetemperature at a rate of 30° C./min from about 35° C. to a maximumtemperature in the range of 300° C. to 420° C., and holding at themaximum temperature for about 4 hours. In some embodiments, calciningcomprises a two-step calcination program wherein the catalyst is heatedto a selected maximum temperature twice, with gradual cooling of thecatalyst between calcinations. In certain embodiments the use ofincreased calcination temperature, ramp rate, and dwell time have asignificant beneficial influence on the crystallinity, pore size,surface area, pore volume, and/or crystallite size. In some embodiments,calcination temperatures greater than 280° C., preferably greater than300° C., assist in creating a more attrition resistant catalyst.

In some embodiments, the nitrate solution comprises Fe²⁺:Fe³⁺ in therange of from about 3%:97% w/w to about 30%:70% w/w and the calcinedcatalyst comprises a maghemite to hematite weight ratio in the range offrom about 1%:99% to about 70%:30%. For example, in some specificembodiments the weight ratio of maghemite to hematite in the calcinedcatalyst is about 30%:70%. In other embodiments the weight ratio ofmaghemite to hematite in the calcined catalyst is about 10%:90%. Incertain embodiments, the calcined catalyst has a maghemite:hematiteweight ratio of about 1%:99%.

The product of an above-described process is also provided, inaccordance with certain embodiments of the invention. In certainembodiments, the catalyst production methods and the resulting catalystshave improved features compared to other Fischer-Tropsch catalysts andproduction methods such as reduced acid amount for iron dissolution,precipitation at low temperatures, and selected precipitation time,selected calcination conditions, mechanical properties, includingattrition resistance, surface characteristics, and enhanced catalyticperformance.

Also provided in accordance with certain embodiments is a catalyst forhydrogenating carbon monoxide, comprising iron, copper and potassium ina weight ratio of 100 Fe:1 Cu:1 K (wt %:wt %:wt %), wherein the iron inthe catalyst comprises a maghemite to hematite weight ratio in the rangeof about 1%:99% to about 70%:30%. The maghemite to hematite weight ratiois determinable qualitatively by peak heights of XRD signals from theprimary peaks of hematite and maghemite at 2θ of 33.1° and 33.6°,respectively, and quantitatively using Mossbauer spectroscopy and/ormagnetic susceptibility measurements. For example, the maghemite tohematite weight ratio is about 30%:70% in some instances, and about10%:90% in other instances, as determinable by equivalent peak heightsof the respective XRD signals.

In some embodiments, the catalyst comprises a particulate structure witha particle size distribution in the range of 10 μm-100 μm. The catalystof this disclosure may comprise a BET surface area in the range of fromabout 45 m²/g to about 150 m²/g. In some embodiments, the catalystcomprises a BET surface area in the range of from about 45 m²/g to about65 m²/g. The catalyst of this disclosure may comprise a mean porediameter in the range of from about 45 Å to about 120 Å. In embodiments,the catalyst comprises a mean pore diameter in the range of from about75 Å to about 120 Å. The inventive catalyst may comprise a mean porevolume in the range of from about 0.20 cc/g to about 0.60 cc/g. In someembodiments, the catalyst comprises a mean pore volume in the range offrom about 0.20 cc/g to about 0.24 cc/g. The catalyst of this disclosuremay comprise a mean crystallite size in the range of from about 15 nm toabout 40 nm. In some embodiments, the catalyst comprises a meancrystallite size in the range of from about 25 nm to about 29 nm.

Also provided in accordance with certain embodiments is a process forhydrogenating carbon monoxide to form a Fischer-Tropsch hydrocarbonproduct. The process includes providing an above-described catalyst,activating the catalyst by exposure to a gas comprising carbon monoxide,hydrogen, or a combination thereof for a selected period of time atselected levels of pressure, temperature, and space velocity, to producean activated catalyst; and contacting a synthesis gas stream with theactivated catalyst in a Fischer-Tropsch slurry-bed reactor whereby aFischer-Tropsch hydrocarbon product is obtained. These and otherembodiments, features and advantages of the present invention will beapparent with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the preferred embodiment of thepresent invention, reference will now be made to the accompanyingdrawings, wherein:

FIG. 1 is a plot of calculated nitric acid to iron weight ratios versusweight percent of ferrous iron in the nitrate solution.

FIG. 2 is a plot of iron concentration in the nitrate solution versusweight percent of ferrous iron in the solution.

FIG. 3 is a plot of measured carbon monoxide conversion versus weightpercent of ferrous iron in the nitrate solution.

FIG. 4 is a plot of measured XRD peak heights of maghemite and hematiteversus weight percent of ferrous iron in the nitrate solution for acalcination temperature of 380° C.

FIG. 5 is a plot of measured XRD peak heights of maghemite and hematiteversus weight percent of ferrous iron in the nitrate solution for acalcinations temperature of 300° C.

NOTATION AND NOMENCLATURE

In the following discussion and in the claims, the terms “comprising,”“including” and “containing” are used in an open-ended fashion, and thusshould be interpreted to mean “including, but not limited to . . . ”.

The singular forms “a,” “an,” and the include plural referents unlessthe context clearly dictates otherwise.

The term “about,” when used in the context of a numerical value, meansapproximately or reasonably close to the given number, and generallyincludes, but is not limited to, ±10% of the stated number.

“Raw” catalyst refers to a formed, dry catalyst after calcination.

The term “activation” refers to the process whereby the raw catalyst istreated using a gas containing carbon monoxide, hydrogen, or acombination thereof for a period of time under certain levels ofpressure, temperature, and space velocity, such that the catalyst isactive for catalyzing the hydrogenation of carbon monoxide to formhydrocarbon products.

The term “space velocity” is defined as the volumetric flow rate ofsynthesis gas (a mixture of hydrogen and carbon monoxide) measured innormal liters per hour divided by the weight of iron in the catalystcontained in the reactor measured in grams.

The term “normal” applies to gaseous material at a temperature of 0° C.and a pressure of 1 atmosphere.

DETAILED DESCRIPTION

Various embodiments of the new iron catalysts are produced by methods inwhich a reduced amount of acid is used for dissolution of the ironstarting material during manufacture of the catalyst than has beenconventionally used in the past. This allows for an increased Fe²⁺: Fe³⁺ratio compared to existing methods. This increased Fe²⁺:Fe³⁺ ratiopermits an increased ratio of lepidocrocite (γ-FeOOH) and/or magnetite(Fe₃O₄) relative to goethite (α-FeOOH) and/or ferrihydrite (Fe₅HO₈.4H₂O)in the precipitated catalyst precursor. Upon heating, lepidocrociteand/or magnetite forms maghemite (γ-Fe₂O₃) and goethite and/orferrihydrite forms hematite (α-Fe₂O₃).

Without wishing to be limited to any particular theory, it is thoughtthat the presence of maghemite in the catalyst creates more defects inthe crystal lattice which can act as active sites. The low temperatureprecipitation allows for greater control over pH during theprecipitation procedure and also allows for improved copper retention,an increase in crystallinity, an increase in pore size, a decrease insurface area, a decrease in pore volume, and/or a decrease incrystallite size. Calcination temperature, ramp rate, and dwell timealso have a significant influence on the crystallinity, pore size,surface area, pore volume, and/or the crystallite size.

Catalyst Manufacturing Process:

The manufacturing process may include the following basic stages: (1)Preparation of nitrate solution containing iron and, in someembodiments, copper nitrates; (2) Preparation of precipitating agent;(3) Heating the solutions; (4) Precipitation; (5) Ripening; (6)Filtering and Washing; (7) Aging; (8) Chemically promoting (Alkalizing);(9) Drying and sizing; and (10) Calcining. Each of these stages is moreparticularly described as follows:

1. Preparation of Nitrate Solution Containing Iron and Copper Nitrates.

The catalyst preparation method comprises preparing an iron nitratesolution having a Fe²⁺:Fe³⁺ ratio (wt %/wt %) in the range of from about0.01% Fe²⁺:99.99% Fe³⁺ to about 100% Fe²⁺:0% Fe³⁺ and optionallycomprising copper. This may be effected by mixing together a selectedamount of metallic iron or an iron-containing compound and a selectedamount of metallic copper in a selected amount of nitric acid having aspecific gravity greater than 1.01 and less than 1.40. A more preferablerange is between 1.050 and 1.100 and a most preferable value is 1.080.The ratio of moles of acid to the moles of iron required for specifiedfractions of ferric nitrate in the solution can be calculated based onstoichiometric balances for the reactions between nitric acid and ironto produce ferric nitrate and ferrous nitrate. The two equations are:Ferric: 4HNO₃+Fe→Fe(NO₃)₃+NO+2H₂OFerrous: 2.5HNO₃+Fe→Fe(NO₃)₂+0.25NH₄NO₃+0.75H₂O

By multiplying the ferric equation by the fraction of ferric iron (f)and the ferrous equation by the fraction of ferrous iron (1-f), andadding the two equations, one obtains the following equation:(2.5+1.5f)HNO₃+Fe→fFe(NO₃)₃+(1−f)Fe(NO₃)₂+fNO+0.25(1−f)NH₄NO₃+(0.75+1.25f)H₂OAs can be seen in the above equation, the ratio of moles of nitric acidto iron is 2.5+1.5f where f is the fraction of ferric ions in thesolution. The weight ratios of acid to iron are determined bymultiplying the molar ratio by the ratio of molecular weights of nitricacid to iron, i.e., 63.02:55.85. In FIG. 1, values of weight ratios ofnitric acid to iron are plotted vs. weight percent of ferrous iron inthe nitrate solution. If it is assumed that a nitric acid solutionhaving a constant specific gravity is used to dissolve the iron over theentire range of ferrous percentages from 0% to 100%, then one canestimate the iron concentration of the nitrate solution. In FIG. 2, theiron concentration is plotted versus the fraction of ferrous iron for anitric acid specific gravity of 1.080. The concentration of nitric acidin the 1.080 specific gravity solution is 14.3% by weight.

According to the analysis presented above, the ratio of weight of acidto the weight of iron should be between 2.8 and 4.5. At values less than2.8, there is insufficient acid to dissolve the iron. At values greaterthan 4.5, there is excess acid beyond that required for dissolution ofthe iron. A preferred range is between 3.0 and 4.0, and more preferablythe value is about 3.55.

Iron from a suitable source is dissolved in the aforementioned nitricacid solution. A suitable grade of iron is one that contains less than100 ppm by weight of sulfur, and less than 10 ppm of chlorine. The ironcan be in powder form or in the form of ingots. The nitrate solutionprepared using the aforementioned nitric acid contains both ferrous andferric ions.

An alternative method for preparing an iron nitrate solution having bothferrous and ferric ions is to prepare the ferrous nitrate and ferricnitrates separately and then mix them together. That is, in embodiments,preparing the iron nitrate solution having a Fe²⁺:Fe³⁺ ratio (wt %/wt %)in the range of from about 0.01% Fe²⁺:99.99% Fe³⁺ to about 100% Fe²⁺:0%Fe³⁺ comprises preparing a ferrous nitrate solution by adding metalliciron or an iron-containing compound and a select amount of nitric acidhaving a specific gravity less than 1.035 and a ferric nitrate solutionby adding metallic iron or an iron-containing compound and a selectamount of nitric acid having a specific gravity greater than 1.115 andcombining them to provide an iron nitrate solution having a specificgravity greater than 1.01 and less than 1.40. In alternativeembodiments, the nitrate solution comprises 100% Fe²⁺.

In embodiments, the ferrous nitrate solution has a nitric acidconcentration of less than about 6 wt %. In embodiments, the ferrousnitrate solution has a nitric acid concentration in the range of fromabout 4 wt % to about 10 wt %. In embodiments, the ferric nitratesolution has a nitric acid concentration of greater than about 20 wt %.In embodiments, the ferric nitrate solution has a nitric acidconcentration of greater than about 25 wt %. In embodiments, the ferricnitrate solution has a nitric acid concentration in the range of fromabout 20 wt % to about 30 wt %.

By using a nitric acid solution having a specific gravity less than1.035, ferrous nitrate only can be produced. By using a nitric acidsolution having a specific gravity greater than 1.115, ferric nitrateonly can be produced. This method can give a more precise ratio offerrous to ferric ions. However, since ferrous nitrate oxidizes readilyto the ferric state, storage of the ferrous nitrate presents a problem.However, the time required for this oxidation is dependent on pH,temperatures and the concentration of other soluble ions. The lower thepH and temperature, the longer time required for the completion of theoxidation reaction. For example, in certain experiments carried out atpH 7.0, the oxidation of Fe²⁺ required 1 hour at 21° C., and required 10hours at 5° C. At pH 6 and at 5° C., the solution required 100 hours tocompletely oxidize.

In embodiments, the ratio of percent Fe²⁺ to percent Fe³⁺ in the ironnitrate solution is in the range of from about 3%:97% to about 30:70(w/w) and the resulting catalyst comprises maghemite and hematite. Insome embodiments, the ratio of Fe²⁺ to Fe³⁺ is about 25%:75% (w/w). Inspecific embodiments, the percentage weight ratio of Fe²⁺ to Fe³⁺ isabout 3.3%:96.7%. In other specific embodiments, the percentage weightratio of Fe²⁺ to Fe³⁺ is about 10%:90%. In certain embodiments, theFe²⁺:Fe³⁺ weight ratio in the resulting nitrate solution after theheating is about 30%:70%.

Copper may be added to the catalyst in several different ways: (1)copper metal from a suitable source can be added to the iron anddissolved in the same nitrate solution prior to precipitation; (2)copper nitrate solution can be prepared separately and added to the ironnitrate solution prior to precipitation; (3) copper nitrate may be addedto the precipitate after precipitation, and prior to spray drying; or(4) copper may be added using any combination of (1) through (3).

The copper preferably contains no more than 1% impurities. The copperacts as an activation promoter in the catalyst. The weight ratio ofcopper to iron is preferably between 0.002 to 0.02, more preferablybetween 0.005 and 0.015, and most preferably between 0.0075 and 0.01. Ifthere exists significant sedimentation or cloudiness in the nitratesolution, the solution may be filtered to remove solids from thesolution. In embodiments, as discussed further in (9) hereinbelow,copper is added as copper nitrate just prior to spray drying. The coppernitrate may be added after precipitation and filtration. In embodiments,the copper nitrate is added with chemical promoter, as described in (8)hereinbelow.

2. Preparation of Precipitating Agent (Chemical Base) Solution.

A 2-10 M aqueous solution of a suitable precipitating agent is prepared.Suitable agents include, but are not limited to, NH₄OH, Na₂CO₃, NaOH,K₂CO₃, KOH, (NH₄)₂CO₃, (NH₄)HCO₃, NaHCO₃ and KHCO₃.

3. Heating the Solutions.

The precipitating agent solution (base solution; e.g., ammoniumhydroxide) and the iron nitrate solution are separately brought totemperatures in the range of ambient to near boiling. For example, insome instances the temperature is in the range of 20° C.-75° C. Thetemperatures of the nitrate solution and the precipitating agentsolution may be the same or different. In some instances, thetemperature of the precipitating agent solution is 25° C., for example.

The catalyst preparation method comprises heating the iron nitratesolution to a temperature in the range of about 20° C. to 80° C. Inother embodiments, the iron nitrate solution is heated to a temperaturein the range of from about 40° C. to about 80° C. In some specificembodiments, the iron nitrate solution is heated to a temperature ofabout 40° C. In other specific embodiments, the iron nitrate solution isheated to a temperature of about 55° C. In some embodiments, thetemperature of the mixture is maintained in the range of 25° C. to 80°C. In embodiments, the iron nitrate solution is heated at a rate oftemperature increase in the range of from 1° C./min to 20° C./min. Incertain embodiments, the iron nitrate solution is heated to about 70° C.at a rate of about 3° C./min. In some instances, the iron nitratesolution is heated to a temperature in the range of 60° C. to 80° C. ata rate of temperature increase in the range of from about 1° C./min toabout 20° C./min. In some instances, the solution is heated to atemperature of 70° C. at a rate of increase of about 3° C./min. Afterheating, the resulting iron nitrate solution has a Fe²⁺:Fe³⁺ ratio inthe range of about 0.01%:99.99% to about 100%:0% (w/w). In somepreferred embodiments, the resulting iron nitrate solution has aFe²⁺:Fe³⁺ ratio in the range of about 3%:97% to about 80%:20% (w/w). Inother preferred embodiments, the resulting iron nitrate solution has aFe²⁺:Fe³⁺ ratio in the range of about 3%:97% to about 30%:70% (w/w).

4. Precipitation

The catalyst preparation method comprises reducing the temperatures ofthe iron nitrate solution and the precipitation agent solution torespective temperatures in the range of 25° C. to 35° C., to obtainrespective low temperature solutions, and precipitating a precipitatecomprising Fe²⁺ and Fe³⁺ phases (e.g., hydroxides) and, in certainembodiments, copper phase (e.g., hydroxide) by reacting the lowtemperature nitrate solution with the low temperature precipitatingagent at a temperature not exceeding 40° C.

In embodiments, the base solution (precipitating agent solution), at atemperature in the range of ambient to near boiling, is gradually addedto the iron nitrate solution to carefully precipitate the iron. In someinstances the temperature of the iron solution is about 35° C. and thetemperature of the base solution is about 25° C., for example. The pH ofthe mixture after precipitation ranges from 6.5 to 9.0. For example, insome instances the precipitation pH is 7.1. Preferably, the basesolution is gradually added to the nitrate solution. For example, thebase solution is added to the nitrate solution over a period of 5 to 180minutes. In some instances, the base solution is added gradually over a20-120 minute period.

The low temperature precipitation allows for greater control over pHduring the precipitation procedure than was possible with most othercatalyst preparation methods in which the temperatures are close to theboiling points of the nitrates and the base. The low temperatureprecipitation also allows for improved copper retention, an increase incrystallinity, an increase in pore size, a decrease in surface area, adecrease in pore volume, a decrease in crystallite size and or acombination thereof in the resulting catalyst particles.

5. Ripening.

The time period between the end of precipitation to the start of washingthe iron hydroxide gel is referred to as “ripening.” The particlesformed during precipitation can continue to grow and change with time aslong as they remain in the liquid from which they precipitated. Thechanges brought about by ripening are beneficial in increasing thecrystallinity of the raw catalyst. In some embodiments, the catalystpreparation method further comprises ripening the precipitate.Preferably the ripening time is in the range of about 30 minutes toabout 60 minutes.

6. Filtering and Washing.

In some embodiments, the catalyst preparation method further compriseswashing the precipitate. In embodiments, the catalyst preparation methodfurther comprises washing the resulting ripened precipitate. Theprecipitated mixture comprising iron hydroxides, goethite and/orferrihydrite and lepidocrocite and/or magnetite and, in someembodiments, copper hydroxides, is filtered and washed to removeresidual nitrates. The slurry containing the precipitate may be firstpumped from the precipitation vessel into a holding tank locatedupstream of a vacuum drum filter. The precipitate (catalyst precursor)is allowed to settle in the holding tank, and a clear layer of nitratesolution forms above the solids. This layer is drawn off before theslurry is washed and filtered. A vacuum drum filter fitted with waterspray bars may be used for washing the catalyst precursor andconcentrating the slurry. To determine when the nitrates have beenremoved from the catalyst precursor, the conductivity of the filtrate ismonitored. The conductivity of the wash water is preferably less than 40micro mhos and more preferably less than 20 micro mhos. Alternatively,the pH of the filtrate can be used to determine the complete removal ofnitrates.

7. Aging.

In some embodiments, the catalyst preparation method comprises aging theripened precipitate or the washed precipitate. Aging may comprises agingfor a period of time ranging from 10 minutes to 30 days. In embodiments,aging may comprises aging for a period of time ranging from 10 minutesto 240 minutes. In embodiments, the washed filter cake (catalystprecursor) obtained from the washing (6) is allowed to age, preferablyfor a period of time between 10 minutes and 240 minutes. More preferablythe filter cake is aged for 30 minutes.

8. Chemically Promoting (Alkalizing).

In some embodiments, the catalyst preparation method comprises slurryingthe precipitate and adding a chemical promoter. In embodiments, thechemical promoter comprises a potassium compound selected from the groupconsisting of K₂CO₃, KHCO₃, and KOH. The weight ratio of potassium toiron in the slurry may be between 0.5 K:100 Fe and 1.5 K:100 Fe, forexample. In some embodiments, the catalyst precursor, comprising theiron and copper hydroxide solids obtained from (6), is slurried in apotassium-containing alkaline solution, preparatory to forming the rawcatalyst particles. The weight ratio of potassium to iron is preferablybetween 0.005 and 0.015, more preferably between 0.0075 and 0.0125, andmost preferably between 0.008 and 0.010.

As mentioned hereinabove, all or a portion of copper may be added ascopper nitrate at this stage or subsequently. In embodiments, copper isadded as copper nitrate solution after precipitation and filtration, butprior to spray drying.

9. Drying and Sizing.

The catalyst method may comprise drying the slurry to form a catalystprecursor. In certain embodiments, the catalyst precursor comprisesparticles having a size distribution of from about 10 microns to about100 microns.

In some embodiments, within preferably 24 hours of preparing the finalsolids slurry in (8), the potassium-containing slurry is spray dried toform spherical particles. In some embodiments, copper is added as coppernitrate just prior to spray drying.

The spray dried particles preferably have a size distribution between 1and 50 microns in diameter, with an average size of 30 microns. Morepreferably, less than 10% by weight of the particles are smaller than 45microns and less than 10% by weight of the particles are larger than 100microns. In embodiments, the median particle diameter is in the range offrom about 60 microns to about 90 microns, and in some embodiments themedian diameter is in the range of from about 70 microns and about 80microns. Air classification of the dried catalyst may be used to achievethe desired particle size distribution. The dried particles preferablyhave a moisture content less than 20% by weight and more preferably lessthan 10% by weight. Alternate means may be used for drying and sizingthat will produce like particles.

10. Calcining.

The catalyst preparation method may comprise calcining the catalystprecursor. In embodiments, the dried catalyst particles from (9) arecalcined at a temperature in the range of 300° C. to 420° C., withgradual ramping of the temperature from ambient temperature. In someinstances, the temperature is increased to the calcining temperature ata rate between 0.5° C./min and 80° C./min. More preferably the ramp rateis between 5° C./min and 50° C./min, and most preferably between 10 and40° C./min.

After the calcining temperature has been attained, the temperature ispreferably held for a time period. In embodiments, the catalyst ismaintained at the calcination temperature for a dwell time period in therange of from about 0.5 hour to about 24 hours. In embodiments, a rotarycalciner is utilized, and the calcination dwell time is from about 0.5hour to about 1.5 hours. In certain embodiments, the dwell time is inthe range of from about 3 to about 6 hours. In other embodiments, thedwell time is a time in the range of from about 4 hours to about 5hours. In embodiments, the dried catalyst particles are calcined for upto 16 hours. In some embodiments, the dwell time is about 24 hours.

Without wishing to be limited by theory, it is postulated that calciningremoves tightly bound water from the particles transforming goethite(α-FeOOH) and/or ferrihydrite (Fe₅HO₈.4H₂O) into hematite (α-Fe₂O₃) andtransforming lepidocrocite (γ-FeOOH) and/or magnetite (Fe₃O₄) intomaghemite (γ-Fe₂O₃). The calcining imparts strength to the particles.

In some instances a two-step calcination program is carried out. Forexample, two passes are made in a rotary calciner to simulate rapid heatup in a fluidized bed. The temperature is first calcined at 319° C. for0.5 hours, ramping from ambient temperature at a rate of increase of 10°C./min, followed by calcining at 319° C. for 8 hours, with ramping at0.5° C./min from ambient. A multi-step calcining program such as this isbelieved to broaden the pore diameter of the particles. The calcinedcatalyst is referred to as raw catalyst.

EXAMPLES

A series of experiments was carried out to determine the pH stability offerrous/ferric nitrate solutions. In these studies iron powder (Hoganas)and nitric acid (VWR, 68-70%) were used to make iron nitrate solutions.The pH of each solution was measured at room temperature on a regularbasis. The following test solutions were prepared:

Ferrous nitrate (Fe²⁺ Nitrate): A ferrous nitrate solution was preparedin an ice bath by dissolving iron powder in nitric acid of specificgravity 1.03122, which corresponds to a nitric acid concentration of 6%by weight. The color of the solution was dark green.

Ferric nitrate (Fe³⁺ Nitrate): A ferric nitrate solution was prepared bydissolving iron powder in nitric acid of specific gravity 1.1469, whichcorresponds to a concentration of nitric acid of 25% by weight. Thecolor of the solution was green indicating that perhaps not all of theiron was in the ferric state.

Preparation of ferrous (25 wt %)/ferric nitrate (75 wt %) solution(Fe²⁺/Fe³⁺ Cold). A ferrous (25 wt %)/ferric nitrate (75 wt %) solutionwas prepared by mixing appropriate amounts of ferrous and ferric nitratesolutions at room temperature. The color of the solution was green.

Preparation of ferrous (25 wt %)/ferric nitrate (75 wt %) solution(Fe²⁺/Fe³⁺ Hot): A ferrous nitrate (25 wt %)/ferric nitrate (75 wt %)solution was also prepared by mixing ferrous and ferric nitratesolutions. Prior to mixing at room temperature, the ferric nitratesolution was heated to 65° C. to make sure all of the iron was ferric.The color of the ferric nitrate solution turned to red while nitricoxide was given off. After mixing the two nitrate solutions, the colorof the solution was green.

The time required for oxidation of ferrous nitrate to ferric nitrate inthe presence of air is dependent on pH, temperature and the presence ofother soluble ions. The lower the pH and temperature, the longer timerequired for the completion of the oxidation reaction. If dissolvedoxygen is present at pH levels above about 7.0, oxygen can be anelectron acceptor according to the following equation:Fe²⁺+1/4O₂+H⁺→Fe³⁺+1/2H₂OIn an acidic environment, the nitrate can be an electron acceptor:Fe²⁺+4/3H⁺→Fe³⁺+1/3NO+1/3H₂O

The pH of each nitrate solution was measured with time and is listed inTable 1. While the pH of each nitrate solution changed, the color of thesolutions also slowly changed to red from green in about a week althoughit is hard to observe gradual change by eye. As the ferrous nitrate wasoxidized, yellow “oxides” were precipitated.

In embodiments, yellow sediments comprising lepidocrocite were observedin the bottom of the ferrous nitrate (Fe²⁺ nitrate) solution in lessthan 24 hours. Analysis by XRD revealed the sediments to compriselepidocrocite and ferrihydrite in some instances. The results from thisexperiment show that ferrous iron tends to go to ferric iron while pHchanges from 5 to about 1.6. This decrease in pH may be due to thegradual hydrolysis of ferrous ions to produce FeOOH and H⁺ in thepresence of air. The change in pH was very fast at the beginning, andslowed down at lower pH values (see Table 1).

TABLE 1 pH Values of Nitrate Solutions pH Time, h Fe²⁺NitrateFe³⁺Nitrate Fe²⁺/Fe³⁺ Cold Fe²⁺/Fe³⁺ Hot 3.00 5   0.3 0.5 1.57 18.00 3¹   0 0.4 1.72 27.00 2.8  0 0.4 1.65 126.00 2.42 0 0.47 1.49¹ 149.00 2.430 0.62 1.51 173.50 2.34 0.17 0.76 1.42 196.50 2.26 0.31 0.94 1.35 270.252.25 0.89 1.26 1.45 292.50 2.18 0.77 1.24 1.36 318.50² 2.08 0.72 1.21.28 434.75 2.09 0.8 1.36 1.36 458.75 2.1  0.86 1.39 1.38 483.75 2.090.98 1.46 1.45 937.00 1.63 0.52 1.02 1.07 963.50 1.82 0.75 1.28 1.22988.00 1.78 0.74 1.27 1.19 1013.00 1.84 0.81 1.31 1.25 1034.50 1.68 0.671.18 1.12 1106.75 1.59 0.63 1.11 1.04 ¹Sediments observed in the bottomof the solution bottle. ²Color of the solutions turned to red.

The pH of Fe³⁺ nitrate solution was very close to zero, and in preferredembodiments was steady for at least a week. The color of the solutionchanged to red from green with time. This may be explained by ionicequilibrium in the solution.

In the ‘Cold’ case, ferrous nitrate and ferric nitrate solutions weremixed at room temperature. Ferrous (25 wt %)/ferric nitrate (75 wt %)solution (Fe²⁺/Fe³⁺ Cold) also showed a change in pH. The color of thesolution changed to red with time, and no sediments were observed.Specific gravity of this solution was calculated to be 1.1179 which isclose to the 1.115 value at which ferric nitrate alone can be formed. Itappeared that ferrous nitrate in the solution was converted to ferricnitrate without leaving any deposit behind, suggesting that theoxidation occurred via oxygen in the air.

In the “Hot” example, ferrous nitrate and ferric nitrate solutions wereprepared using 6% and 25% nitric acid solutions, the same as in theFe²⁺/Fe³⁺ ‘Cold’ case. However, ferric nitrate solution was heated to65° C. to make sure all the iron is in ferric (Fe³⁺) state. The ferricnitrate solution was then cooled to room temperature and mixed withferrous (Fe²⁺) nitrate solution at room temperature. Ferrous (25 wt%)/ferric nitrate (75 wt %) solution (Fe²⁺/Fe³⁺ Hot) showed changes inpH which was around 1.5 at the beginning, and decreased to 1 in time.Precipitation of ferric hydroxide was observed after the first 300 hoursand slowly increased in time. Heating ferric nitrate prior to mixingferrous and ferric nitrate solutions may have created a media that canform ferric hydroxide by hydrolysis reaction, which is dependent on theconcentration, time, temperature, acidity and the presence of othersubstances in the solution. Oxidation of ferrous to ferric occurs at amuch lower rate at lower pH and any insoluble ferric hydroxide formed isnot precipitated as readily at low pH.

Inventive catalysts formed from nitrate solutions comprising Fe²⁺:Fe³⁺ratios of 10:90 (Example 1); 100:0 (Example 2); 20:80 (Example 3); 50:50(Example 4); and 80:20 (Example 5); respectively are presented inExamples 1-5 which follow. Example 6 hereinbelow describes a catalystformed from a nitrate solution comprising a Fe²⁺:Fe³⁺ ratio of and 0:100(Example 6) and is presented for comparison with the inventivecatalysts.

Example 1 Preparation of a Copper, Potassium Promoted Iron CatalystPrepared with ˜10% Ferrous/90% Ferric Nitrate Solution (InventiveCatalyst)

This example delineates the steps in the preparation of a representativeraw catalyst. The following reagents were employed: iron powder(Höganäs, 98.61% Fe, −325 mesh); copper powder, (Alfa Aesar, −40+100mesh), 99.5% metals basis; potassium carbonate, K₂CO₃ (Alfa Aesar), ACSreagent grade; nitric acid, 70% (Fisher), certified ACS PLUS grade;ammonium hydroxide, (EMO) 160 mL, ACS reagent grade; and deionized (DI)water. The catalyst was prepared according to the following procedure:

1. 20.186 g iron powder and 0.200 g copper powder were slurried with 100mL DI water to prevent hot spots due to exothermic reaction during thedissolution process.

2. Nitric acid (100.8 g of 70% HNO₃) was dissolved in 302 mL DI water.Reduced acid amounts will produce excessive maghemite in the final oxideproduct.

3. With mechanical stiffing, the nitric acid solution was added to theiron slurry dropwise over 75 minutes. The addition rate was such thatthe temperature of the iron solution did not go above 35° C. LightNO_(x) evolution is observed above 40° C.

4. After complete addition of the nitric acid solution the dark greeniron solution was stirred until the iron was completely dissolved.

5. This “iron and copper nitrate” solution was then heated to 70° C. atabout 3° C./min. Above 60° C. (65-70° C.), NOx gases with reddish browncolor were produced. During this heating period the color of the mixturechanged from dark green to red/brown. The Fe²⁺:Fe³⁺ weight ratio of thenitrate solution is about 10/90.

6. A 14.5% ammonium hydroxide solution was prepared by combining equalvolume portions of 29% ammonium hydroxide and DI water.

7. The base solution was added slowly to the Fe/Cu nitrate solution (35°C.) over 60 minutes while monitoring the pH of the solution. At pH 2.5to 4, a voluminous precipitate formed and the stirring becameinefficient. The addition of the base was stopped temporarily to regainstirring efficiency. Base addition was then continued until the pHreached 7.1±0.1.

8. The mixture was then stirred at 35-25° C. (no heating necessary) for30 minutes while a pH of 7.1±0.1 was maintained. A sample of the mixturewas obtained.

9. The mixture was filtered and washed three times with 1000 mL ofwater. The pH of the filtrate was monitored with a pH meter, firstfiltrate pH about 7.15, second filtrate (1st wash) pH about 7.10, thirdfiltrate (2nd wash) pH about 7.05, forth filtrate (3rd wash) pH about6.6. The residue was washed further until the pH did not change.Preferably a water conductivity number of below 40 micro mho, morepreferably below 20 micro mho is measured. Alternatively, anothersuitable method may be used to measure residual nitrates.

10. The filter residue was dried sufficiently so that it was easilyremoved from the filter paper, but not so that it was totally dry.

11. The filtered residue was slurried with 0.352 g potassium carbonatedissolved in 10 mL of DI water to generate an 11-12 wt % solids mixture.

12. Four batches of slurry were prepared according to the proceduredescribed in steps 1 through 12 and mixed together. This mixture wasspray dried to spherical particles using a Type H Mobile Niro spraydryer consisting of a two-fluid nozzle atomizer, drying chamber, airdisperser, chamber, product collection section, air ducts, cyclone,exhaust fan, air heater, and instrument panel. Using the Type H MobileNiro spray dryer, the “feed” was introduced through a nozzle from thebottom with the drying air cross flowing from the top under thefollowing conditions: Inlet Temperature: 370° C. (±2); OutletTemperature: 105° C. (±2); Slurry Solids Content: 11% (±1); Water SetupFlow 4.0 to 4.5 kg/hr (feed flow is set with water, and then switched toactual feed slurry); and Atomizer Air Flow at 1 bar pressure set between2 and 6 kg/h, more preferably between 3 and 5 kg/h and most preferablybetween 3 and 4 kg/h.

13. The spray dried material was then calcined by heating to 300° C. at30° C./min and holding at that temperature for 16 hours,

Catalyst Physical Properties

The preferred properties of the catalysts made according to theprocedure of Example 1 will be described in this section.

In embodiments, the catalyst has ratios of 100 Fe/1 Cu/1 K (w/w/w). Thistheoretically corresponds to 68.496% Fe, 0.685% Cu and 0.685% K, byweight.

In embodiments, the catalyst particles are spherical in shape, and theparticle size distribution is in the range of 10 μm±100 μm.

The BET surface area of the catalyst may be in the range of from about45 m²/g to about 85 m²/g. The BET surface area of the catalyst may, inembodiments, be in the range of from about 65 m²/g to about 85 m²/g.

The pore diameter of the catalyst produced according to Example 1 may bein the range of from about 75 Å to about 120 Å.

The pore volume of the catalyst produced according to Example 1 may bein the range of from about 0.20 cc/g to about 0.24 cc/g.

The XRD of the catalyst according to Example 1 may have signals from theprimary peaks of Hematite and Maghemite (2θ of 33.1° and 33.6°), withapproximately equivalent peak heights.

The catalyst may have a crystallite size in the range of from about 25nm to about 9 nm.

The catalyst may exhibit a maximum reduction by TPR at temperaturesbetween 250° C. and 275° C. In some embodiments, the catalyst mayexhibit a maximum reduction by TPR at temperatures between 250° C. and265° C.

The Loss on Ignition (LOI) of the catalyst may be less than 6%.

Elemental Analysis of catalyst according to Example 1 may show copper inthe range of from about 0.58% to about 0.68% and potassium in the rangeof from about 0.67% to about 0.77%, as analyzed by Atomic AdsorptionSpectroscopy.

The catalyst activity over 500 hours on stream may provide a COconversion greater than 80%. The carbon dioxide selectivity may bebetween 37% and 43% by volume. The methane selectivity is preferablybetween 0.8% and 1.2% by volume. The catalyst deactivation rate ispreferably less than 1% per week. Standard activation and reactionconditions for catalyst performance were: Activation: 275° C., 140 psig,space velocity: 2.5 nl/g Fe/h, H₂/CO=1.4; Reaction: 255° C., 375 psig,space velocity 3.45 nl/g Fe/h, H₂/CO=0.77.

In embodiments, the purity of the catalyst is as shown in Table 2. It ispossible that the purity of the final catalyst may be improved byselecting a higher purity iron source.

TABLE 2 Catalyst Contaminants Contaminant ppm Nitrate <5000 Sulfur <500Chlorine <200 Aluminum <700 Calcium <50 Chromium <250 Magnesium <500Manganese <1500 Silicon <1500 Sodium <50 Titanium <1500 Vanadium <1000Zinc <200 Phosphorus <100 Nickel <400 Cobalt <100 Lead <50Analytical Methods Used to Determine Catalyst Properties:

X-Ray Diffraction Analysis (XRD). X-ray diffraction analysis was carriedout using the following scan parameters: Range (2θ) 7.0100 to 89.9900;Step size (2θ) 0.0200; Time per step (s) 0.35; Number of data points4150; Minimum (counts/sec) 0.00; Maximum (counts/sec) 1331; Scan modeContinuous; Diffractometer, Configuration and Settings: Control unitPW3710, Goniometer PW1050, Generator PW1830/00, Generator tension (kV)40, Generator current (mA) 40, X-ray tube PW2773 Cu Long Fine Focus,Tube focus Line, Take off) angle(°) 6.0000, Divergence slit Fixed slit1°, Incident beam radius (mm) 173.00 Incident bead soller slit 0.04 rad,Diffracted beam radius (mm) 173.00, Receiving slit height, Fixed slit0.2 mm, Detector PW3011,

BET Surface Area. Analysis was performed using a Quadrachrome NOVA 2000eor a Quadrachrome Quadrasorb instrument. Surface areas and pore sizeswere determined from multi-point nitrogen volume/partial pressureisotherms using the BET method. Pore diameters were determined using BJHdesorption dv method. Samples were vacuum degas sed at 100° C. for 4hours.

Adsorption points: P/Po=0.050000, 0.009167 M, 0.013330 M, 0.017500 M,0.021667 M, 0.025833 M, 0.030000 M, 0.032500, 0.106938, 0.180577,0.254615, 0.328654, 0.402692, 0.476731, 0.550769, 0.624808, 0.698846,0.772885, 0.896923, 0.920962.

Desorption points: P/Po=0.995000 V P, 0.995750 P, 0.896500 P, 0.8472550P, 0.798000 P, 0.748750 P, 0.699500 P, 0.650250 P, 0.601000 P, 0.55175P, 0.502500 P, 0.453250 P, 0.404000 P, 0.354750 P, 0.305500 P, 0.256250P, 0.207000 P, 0.157750 P, 0.108500 P, 0.059250 P, 0.010000 P.

Temperature-Programmed Reaction (TPR). A 20-25 mg sample was weighed outand placed within the sample tube on top of a quartz wool plug. The tubewas connected to the main instrument housing of a MicromeriticsChemiSorb 2750 w/optional ChemiSoft TPx System using knurled nuts and acompression O-ring. The provided programmable instrument furnace,capable of reaching 1100° C., was placed around the sample tube. Athermocouple was installed through the top of the sample port and downinto the sample tube using a Teflon ferrule and oriented so the tip wasembedded just slightly in the sample mass. The sample was then degassedat 150° C. under a 50 mL/min flow of nitrogen for one hour and thenallowed to cool under that same atmosphere prior to testing. Once thesample had cooled, the nitrogen gas was turned off and the testing gas(10% H₂ in Argon) was turned on and allowed to flow over the sample for15 minutes at 50 mL/min prior to testing. A frozen isopropyl alcoholcold trap was prepared and placed around the cold trap on the instrumentin order to freeze out water generated during testing prior to the testgas running through the thermal conductivity detector (TCD). In oneexperiment the furnace temperature was ramped from room temperature to350° C. at 5° C./min under a test gas flow of 50 mL/min. The furnacetemperature may be ramped to as much as about 450-500° C. Changes in TCDsignal are charted on the instrument software vs. both time andtemperature as registered on the internal thermocouple.

Metals Content (% Cu and % K). These protocols have been applied tocatalyst formulations which are basically 98% iron oxides, 1% copperoxides, and 1% potassium oxide. Other compounds may be present inminimal concentrations as contaminant species.

Digestion Procedure: a) Weigh catalyst sample (100±10 mg) into a 50 mLbeaker with a watch glass cover; b) Add 10 mL of 35% HNO₃; c) Bring to agentle boil for 45 minutes of refluxing. Maintain volume between 5 mLand 10 mL with de-ionized water; d) Cool on lab bench for 1-3 minutes;e) Add 5 mL of concentrated HCl; f) Bring to a gentle boil for 15minutes of refluxing; g) Cool on lab bench for 1-3 minutes; h) Removewatch glass cover and if necessary, return to hot plate to reduce volumeto about 10 mL; and i) Transfer digestate to 100 mL volumetric flask(Class A) and bring to volume with % HNO₃. This is called the diluteddigestate.

Analysis Procedure: a) Prepare calibration standards from certifiedprimary standard; b) Dilute 1.000 mL of diluted digestate into a 10 mLvolumetric flask (Class A) with desired matrix for element of interest.The dilution matrix used is dependent upon the matrix of the primarystandard. This dilution step can be modified to produce samples withconcentrations of analyte within the range of the calibration standards.Potassium samples require an ionization suppressant of 0.1-0.2% CsCl orRbCl; c) Analyze calibrations standards and unknowns by atomicabsorption spectrophotometry using a suitable apparatus such as aShimadzu AA-6501 equipped with a graphite furnace and autosampler.

Crystallite Size. Crystallite Size was calculated using the Full WidthHalf Maximum (FWHM) of the XRD peaks and the Scherrer Equation (1918). Ahighly crystalline hematite sample (Aldrich, >98%, approximately 5 μm)was scanned and the FWHM of its peaks were used in the calculations. Thecrystallite size was calculated for 4 peaks and averaged. The 4 hematitepeaks were at 2θ values of 24.1°, 40.8°, 49.4°, and 51.4°.

The above-described catalyst preparation method allows for control overmajor preparation parameters. The reduced amount of acid for dissolutionof the iron starting material allows for an increased Fe²⁺:Fe³⁺ ratio.In the final raw catalyst product this increased Fe²⁺:Fe³⁺ ratio impartsan increased maghemite (γ-Fe₂O₃):hematite (α-Fe₂O₃) ratio. The increasedmaghemite:hematite ratio in the raw catalyst provides a catalyst havinghigher activity for carbon monoxide conversion than does a raw catalystcomprising hematite only. The low temperature precipitation allows forgreater control over pH during the precipitation procedure and alsoallows for improved copper retention, an increase in crystallinity andpore size, and a decrease in surface area, pore volume, and crystallitesize. Calcination temperature, ramp rate, and dwell time also have asignificant influence on the crystallinity, pore size, surface area,pore volume, and/or crystallite size.

In Table 3 certain data are listed for Example 1.

TABLE 3 Summary of Parameters for Example 1 Process step/componentDescription Iron source metallic iron Copper source metallic copperCopper addition method dissolved with iron HNO₃ specific gravity 1.08sp. gr. HNO₃ (14%) HNO₃ Used 3.5:1 acid:iron ratio Iron Dissolution <35°C. Temperature Iron Solution Temperature 70-35° C. Base 4M NH₄OH BaseTemp. for Precipitation 25° C. Precipitation Temp. 35° C. PrecipitationpH 7.1 Total Time for Precipitation 90 min. Ripen Time 0.5 h Potassiumsource K₂CO₃ Mode of potassium addition slurried before spray dryingDrying technique spray drying Calcination temperature 300° C./16 h; rampat 30° C./min

Example 2 Catalyst Prepared with Ferrous Nitrate Solution Only

This example delineates the steps used in preparing a catalyst usingferrous nitrate only.

1. Iron powder (15 g) was slurried with 114.4 g of DI water to preventhot spots due to exothermic reaction during the dissolution process.

2. Nitric acid, 49.8 g of 68% HNO₃, was dissolved in 400.0 g DI water.

3. With mechanical stiffing, the nitric acid solution was added dropwiseto the iron slurry which was cooled by an ice bath. The temperature ofthe iron solution was maintained below 30° C.

4. After complete addition of the nitric acid solution, the dark greeniron solution was stirred until the iron was completely dissolved.

5. This dark green ferrous nitrate solution was filtered through a finefilter paper.

6. The total volume of ferrous nitrate solution obtained in Step 5 wasabout 550 mL.

7. A 49.3 g of 29% ammonium hydroxide was diluted by DI water to 550 mLat room temperature.

8. The base solution at 22° C. obtained in Step 7 was added slowly tothe ferrous nitrate solution at 22° C. over a 10 minute period whilemonitoring the pH of the solution. Neither solution was heated.Precipitation ended at pH 7.30.

9. The mixture was filtered and washed three times with 1000 mL of waterfor each washing. The precipitate was green in color. The residue waswashed until the pH was near 7.0. The filter cake was dried sufficientlyso that it was easily removed from the filter paper.

10. The filter cake was dried slowly, ground, and dried at 120° C. in anoven overnight.

11. An aqueous solution of 0.5703 g of Cu(NO₃)₂.3H₂O was impregnatedonto the dried and ground powder by the incipient wetness method.

12. The wet powder was dried at 120° C. in an oven overnight.

13. An aqueous solution of 0.2651 g of K₂CO₃ was then impregnated ontothe ground powder by the incipient wetness method.

14. The wet powder was dried at 120° C. in an oven overnight.

15. The ground powder material was placed in an oven and was firstramped to 125° C. at the rate of 2° C./min, and held at 125° C. for 5 h.It was then heated to 300° C. at the same rate, and held at 300° C. for16 h.

Example 3 Catalyst Prepared with 20% Ferrous/80% Ferric Nitrate Solution

Preparation of Ferric Nitrate Solution:

1. The iron powder (8.0 g) was slurried with 8.5 g DI water to preventhot spots due to exothermic reaction during the dissolution process.

2. Nitric acid, 39.8 g of 68% HNO₃, was dissolved in 60.0 g DI water.

3. With mechanical stiffing, the nitric acid solution was added dropwiseto the iron slurry which was cooled by ice bath. The temperature of theiron solution did not exceed 30° C.

4. After complete addition of the nitric acid solution, the light greensolution was stirred until the iron is completely dissolved.

5. The light green iron nitrate solution was filtered through finefilter paper.

6. This iron nitrate solution is then heated to 70° C. at 3° C./min tomake sure all iron in iron nitrate solution in ferric state. Evolutionof NO_(x) gases with reddish brown color was observed. During thisheating period, the color of this mixture changed from a light green toa reddish brown, and ferric nitrate solution was obtained.

7. The ferric nitrate solution was cooled down to room temperature.

Preparation of Ferrous Nitrate Solution:

8. The iron powder (2 g) was slurried with 8.6 g DI water to prevent hotspots due to exothermic reaction during the dissolution process.

9. Nitric acid, 6.6 g of 68% HNO₃, was dissolved in 60.0 g DI water.

10. With mechanical stirring, the nitric acid solution was addeddropwise to the iron slurry which was cooled by ice bath. Thetemperature of the iron solution did not exceed 30° C.

11. After complete addition of the nitric acid solution, the dark greeniron solution was stirred until the iron is completely dissolved.

12. This dark green ferrous nitrate solution was filtered through finefilter paper.

Mixing of Ferrous Nitrate and Ferric Nitrate Solutions:

13. The ferric nitrate solution obtained in Step 7 and the ferrousnitrate solution obtained in Step 12 were mixed at 22° C. The totalvolume of ferrous and ferric nitrates solution was about 300 mL. The pHof the mixture was 2.20.

14. A 32.9 g of 29% ammonium hydroxide was diluted by DI water to 300mL, same volume of nitrates solution at room temperature.

15. The base solution at 22° C. (obtained in Step 14) is added slowly tothe ferrous and ferric nitrates solution at 22° C. (obtained in Step 13)over 10 minutes while monitoring the pH of the solution. Neithersolution was heated. Precipitation ended at pH 7.22.

16. The mixture is filtered and washed three times with 1000 mL of waterfor each washing. The precipitate was dark brown in color. The residuewas washed until pH is near neutral. Alternative methods can also beused to measure nitrates.

17. The filter residue was dried sufficiently so that it was easilyremoved from the filter paper. It was dried slowly, ground, and dried at120° C. in an oven overnight. Aqueous solution of 0.3802 g ofCu(NO₃)₂.3H₂O was impregnated onto ground powder by incipient wetnessmethod, and dried at 120° C. in an oven overnight. Aqueous solution of0.1767 g of K₂CO₃ was then impregnated onto the ground powder byincipient wetness method, and dried at 120° C. in an oven overnight.

18. The ground powder material placed in an oven was first ramped to125° C. at the rate of 2° C./min, held at 125° C. for 5 h, and then itwas heated up to 300° C. at the same rate, and held at 300° C. for 16 h.

Example 4 Catalyst Prepared with 50% Ferrous Nitrate+50% Ferric NitrateSolution

This example lists the steps used in preparing a catalyst using amixture comprising 50% by weight ferrous iron and 50% by weight ferriciron in nitrate solutions prepared separately.

Preparation of Ferric Nitrate Solution:

1. The iron powder (5.0 g) was slurried with 7.8 g DI water to preventhot spots due to exothermic reaction during the dissolution process.

2. Nitric acid, 24.90 g 68% HNO₃, was dissolved in 35.0 g DI water.

3. With mechanical stiffing, the nitric acid solution was added dropwiseto the iron slurry which was cooled by an ice bath. The temperature ofthe iron solution did not exceed 30° C.

4. After complete addition of the nitric acid solution, the light greeniron solution was stirred until the iron was completely dissolved.

5. The light green iron nitrate solution was filtered through finefilter paper.

6. This iron nitrate solution was then heated to 70° C. at 3° C./min tomake sure all iron in the iron nitrate solution was in the ferric state.Evolution of NO_(x) gases with reddish brown color was observed. Duringthis heating period, the color of this mixture changed from a lightgreen to a reddish brown.

7. The ferric nitrate solution was cooled down to room temperature.

Preparation of Ferrous Nitrate Solution:

8. The iron powder (5.0 g) was slurried with 21.4 g DI water to preventhot spots due to exothermic reaction during the dissolution process.

9. Nitric acid, 16.6 g of 68% HNO₃, was dissolved in 150.0 g DI water.

10. With mechanical stiffing, the nitric acid solution was addeddropwise to the iron slurry which was cooled by an ice bath. Thetemperature of the iron solution did not exceed 30° C.

11. After complete addition of the nitric acid solution, the dark greeniron solution was stirred until the iron was completely dissolved.

12. This dark green ferrous nitrate solution was filtered through finefilter paper.

Mixing of Ferrous and Ferric Nitrate Solutions:

13. The ferric nitrate solution obtained in Step 7 and ferrous nitratesolution obtained in Step 12 were mixed at 22° C. The total volume offerrous and ferric nitrates solution was about 260 mL. The pH of themixture was 2.23.

14. A 32.9 g of 29% ammonium hydroxide was diluted by DI water to 260mL, the same volume of the nitrates solution at room temperature.

15. The base solution at 22° C. (obtained in Step 14) was added slowlyto the ferrous and ferric nitrates solution at 22° C. over a 10 minuteperiod while monitoring the pH of the solution. Neither solution washeated. Precipitation ended at pH 7.3.

16. The mixture was filtered and washed three times with 1000 mL ofwater for each washing. Filtration was very slow and the precipitate wasdark in color. The residue was washed until the pH was near 7.0.

17. The filter residue was dried sufficiently so that it was easilyremoved from the filter paper. It was dried slowly, ground, and dried at120° C. in an oven overnight. An aqueous solution of 0.3802 g ofCu(NO₃)₂.3H₂O was impregnated onto the ground powder by the incipientwetness method, and dried at 120° C. in an oven overnight. An aqueoussolution of 0.1767 g of K₂CO₃ was then impregnated onto the groundpowder by incipient wetness, and dried at 120° C. in an oven overnight.

18. The ground powder material placed in an oven was first ramped to125° C. at the rate of 2° C./min, held at 125° C. for 5 h, and then itwas heated up to 300° C. at the same rate, and held at 300° C. for 16 h.

Example 5 Catalyst Prepared with 80% Ferrous/20% Ferric Nitrate Solution

This example lists the steps used in preparing a catalyst using amixture comprising 80% by weight ferrous iron and 20% by weight ferriciron in nitrate solutions prepared separately.

Preparation of Ferric Nitrate Solution:

1. The iron powder (2.0 g) was slurried with 2.1 g DI water to preventhot spots due to exothermic reaction during the dissolution process.

2. Nitric acid, 6.8 g of 68% HNO₃, was dissolved in 15.0 g DI water.

3. With mechanical stiffing, the nitric acid solution was added dropwiseto the iron slurry which was cooled by an ice bath. The temperature ofthe iron solution did not exceed 30° C.

4. After complete addition of the nitric acid solution, the light greensolution was stirred until the iron was completely dissolved.

5. The light green iron nitrate solution was filtered through finefilter paper.

6. This iron nitrate solution was then heated to 70° C. at 3° C./min tomake sure all iron in the iron nitrate solution was in the ferric state.Evolution of NO_(x) gases with reddish brown color was observed. Duringthis heating period, the color of this mixture changed from a lightgreen to a reddish brown.

7. The ferric nitrate solution was cooled down to room temperature.

Preparation of Ferrous Nitrate Solution

8. The iron powder (8.0 g) was slurried with 24.4 g DI water to preventhot spots due to exothermic reaction during the dissolution process.

9. Nitric acid, 26.6 g of 68% HNO₃, was dissolved in 250.0 g DI water.

10. With mechanical stiffing, the nitric acid solution was addeddropwise to the iron slurry which was cooled by an ice bath. Thetemperature of the iron solution did not exceed 30° C.

11. After complete addition of the nitric acid solution, the dark greensolution was stirred until the iron was completely dissolved.

12. This dark green ferrous nitrate solution was filtered through finefilter paper.

Mixing of Ferrous and Ferric Nitrate Solutions:

13. The ferric nitrate solution obtained in Step 7 and ferrous nitratesolution obtained in Step 12 were mixed at 22° C. The total volume offerrous and ferric nitrates solution was about 350 mL. The pH of themixture was 1.76.

14. A 32.9 g of 29% ammonium hydroxide was diluted by DI water to 350mL, the same volume of nitrates solution at room temperature.

15. The base solution at 22° C. (obtained in Step 14) was added slowlyto the ferrous and ferric nitrates solution at 22° C. (obtained in Step13) over 10 minutes while monitoring the pH of the solution. Neithersolution was heated. Precipitation ended at pH 7.26 at 25° C.

16. The mixture was filtered and washed three times with 1000 mL ofwater for each washing. The precipitate was dark in color. The residuewas washed until pH was near 7.0. The filter residue was driedsufficiently so that it was easily removed from the filter paper. It wasdried slowly, ground, and dried at 120° C. in an oven overnight. Anaqueous solution of 0.3802 g of Cu(NO₃)₂.3H₂O was impregnated ontoground powder by incipient wetness, and dried at 120° C. in an ovenovernight. An aqueous solution of 0.1767 g of K₂CO₃ was then impregnatedonto the ground powder by incipient wetness method, and dried at 120° C.in an oven overnight.

17. The ground powder material placed in an oven was first ramped to125° C. at the rate of 2° C./min, held at 125° C. for 5 h, and then itwas heated up to 300° C. at the same rate, and held at 300° C. for 16 h.

Example 6 Catalyst Prepared with Ferric Nitrate Solution Only(Comparative Catalyst)

1. The iron powder (15.0 g) was slurried with 23.4 g DI water to preventhot spots due to exothermic reaction during the dissolution process.

2. Nitric acid, 74.7 g of 68% HNO₃, was dissolved in 105.0 g DI water.

3. With mechanical stiffing, the nitric acid solution was added dropwiseto the iron slurry which was cooled by ice bath. The temperature of theiron solution did not exceed 30° C.

4. After complete addition of the nitric acid solution, the light greeniron solution was stirred until the iron is completely dissolved.

5. The light green iron nitrate solution was filtered through a finecourse filter paper.

6. This iron nitrate solution is then heated to 70° C. at 3° C./min tomake sure all iron in iron nitrate solution in ferric state. Evolutionof NO_(x) gases with reddish brown color was observed. During thisheating period, the color of this mixture changed from a light green toa reddish brown, and ferric nitrate solution was obtained.

7. The ferric nitrate solution was cooled down to 22° C.

8. The total volume of the solution obtained in Example 5, Step 7 wasabout 200 mL.

9. A 49.3 g of 29% ammonium hydroxide was diluted by DI water to 200 mL,same volume of nitrates solution at room temperature.

10. The base solution at 22° C. (obtained in Example 5, Step 9) wasadded slowly to the ferric nitrate solution at 22° C. (obtained inExample 5, Step 8) over 10 minutes while monitoring the pH of thesolution. Neither solution was heated. Precipitation ended at pH 7.39.

11. The mixture was filtered and washed three times with 1000 mL ofwater for each washing. The precipitate was dark in color. The residuewas washed until pH is near neutral. Alternative methods can also beused to measure nitrates.

12. The filter residue was dried sufficiently so that it was easilyremoved from the filter paper. It was dried slowly, ground, and dried at120° C. in an oven overnight. An aqueous solution of 0.5703 g ofCu(NO₃)₂.3H₂O was impregnated onto the ground powder by incipientwetness method, and dried at 120° C. in an oven overnight. An aqueoussolution of 0.2651 g of K₂CO₃ was then impregnated onto the groundpowder by incipient wetness method, and dried at 120° C. in an ovenovernight.

13. The ground powder material placed in an oven was first ramped to125° C. at the rate of 2° C./min, held at 125° C. for 5 h, and then itwas heated up to 300° C. at the same rate, and held at 300° C. for 4 h.

Example 7 Comparison of Catalysts from Examples 2 Through 6

The five catalysts prepared in Examples 2 through 6 were tested fortheir performance in catalyzing the hydrogenation of carbon monoxide(the Fischer-Tropsch reaction) in a 2 liter autoclave (continuousstirred tank reactor).

The raw catalysts were slurried in a synthetic oil and activated in theautoclave using synthesis gas under the following conditions:

H₂:CO Ratio: 1.40 Temperature: 275° C. (No ramp) Pressure: 140 PsigSpace Velocity: 2.4 NL(H₂ + CO)/h/gFe Time: 5-6 h

After activation, the conditions were changed to operating conditions:

H₂:CO Ratio: 0.77 Temperature: 255° C. (No ramp) Pressure: 375 PsigSpace Velocity: 3.1 NL(H₂ + CO)/h/gFeApproximately 10% by volume of N₂ was fed along with the H₂ and CO foruse as an internal standard.

In Table 4 below, values of CO conversion are tabulated at fourdifferent times to show the relative activity and stability of thedifferent catalysts. These results are plotted in FIG. 3.

TABLE 4 CO Conversion Comparisons Catalyst % Ferrous % Ferric @100 h@150 h @ 200 h @ 250 h Example 2 100 0 79.0 78.0 74.5 71.0 Example 3 2080 86.0 85.0 84.0 84.0 Example 4 50 50 90.5 90.5 90.0 88.0 Example 5 8020 88.5 88.0 87.5 87.5 Example 6 0 100 72.0 72.5 70.5 68.0 (ComparativeCatalyst)

FIG. 4 is a plot of measured XRD peak heights of maghemite and hematiteversus weight percent of ferrous iron in the nitrate solution for acalcination temperature of 380° C. FIG. 5 is a plot of measured XRD peakheights of maghemite and hematite versus weight percent of ferrous ironin the nitrate solution for a calcinations temperature of 300° C.

While the preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit and teachings of the invention. Theembodiments described herein are exemplary only, and are not intended tobe limiting. Many variations and modifications of the inventiondisclosed herein are possible and are within the scope of the invention.Accordingly, the scope of protection is not limited by therepresentative description set out above, but is only limited by theclaims which follow, that scope including all equivalents of the subjectmatter of the claims.

The disclosures of all patents, patent applications, and publicationscited herein are hereby incorporated herein by reference in theirentirety, to the extent that they provide exemplary, procedural, orother details supplementary to those set forth herein.

What is claimed:
 1. A method of manufacturing a catalyst comprisingiron, copper and potassium, comprising: preparing an iron nitratesolution having a Fe²⁺:Fe³⁺ ratio (wt %/wt %) in the range of from about0.01% Fe²⁺:99.99% Fe³⁺ to about 100% Fe²⁺:0%Fe³⁺ and comprising copper;heating the iron nitrate solution to a temperature in the range of about20° C. to 80° C.; preparing a precipitating agent solution; reducing thetemperature of the iron nitrate solution to a temperature in the rangeof from about 25° C. to 35° C. to obtain low temperature iron nitratesolution and reducing the temperature of the precipitation agentsolution to a temperature in the range of 25° C. to 80° C. to obtain lowtemperature precipitating agent solution; and precipitating aprecipitate comprising iron phases and optionally copper phases byreacting the low temperature nitrate solution with the low temperatureprecipitating agent at a temperature not exceeding 40° C.
 2. The methodof claim 1 wherein the iron nitrate solution has a Fe²⁺:Fe³⁺ ratio (wt%/wt %) in the range of from about 3% Fe²⁺:97% Fe³⁺ to about 80%Fe²⁺:20% Fe³⁺.
 3. The method of claim 1 wherein preparing the ironnitrate solution comprises preparing a ferrous nitrate solution byadding metallic iron or an iron-containing compound and a select amountof nitric acid having a specific gravity less than 1.035 and a ferricnitrate solution by adding metallic iron or an iron-containing compoundand a select amount of nitric acid having a specific gravity greaterthan 1.115 and combining them to provide an iron nitrate solution havinga specific gravity greater than 1.01 and less than 1.40.
 4. The methodof claim 3 wherein preparing an iron nitrate solution comprisespreparing a copper nitrate solution and an iron nitrate solutionseparately from each other, and combining the separate solutions to formthe iron nitrate solution comprising copper.
 5. The method of claim 1further comprising slurrying the precipitate and adding a chemicalpromoter.
 6. The method of claim 5 further comprising ripening theprecipitate and washing the resulting ripened precipitate to removenitrates prior to slurrying the precipitate.
 7. The method of claim 6further comprising aging the resulting washed precipitate.
 8. The methodof claim 7 wherein aging the washed precipitate comprises aging for aperiod of time ranging from 10 minutes to 240 minutes.
 9. The method ofclaim 7 wherein aging the precipitate comprises allowing the washedcatalyst precursor to stand for a period in the range of from about 10minutes to about 240 minutes.
 10. The method of claim 6 wherein ripeningthe precipitate comprises ripening for a period of time ranging from 30minutes to 60 minutes.
 11. The method of claim 5 further comprisingdrying the slurry to form a catalyst precursor.
 12. The method of claim11 further comprising calcining the catalyst precursor to form a rawcatalyst.
 13. The method of claim 12 further comprising activating theraw catalyst by exposure to a gas comprising carbon monoxide, hydrogen,or a combination thereof for a selected period of time at selectedlevels of pressure, temperature, and space velocity sufficient toenhance catalytic activity for hydrogenating carbon monoxide to formhigher hydrocarbons.
 14. The method of claim 12 wherein the resultingcatalyst precursor comprises oxide particles having a size distributionfrom 10 microns to 100 microns.
 15. The method of claim 12 whereincalcining the catalyst precursor comprises calcining according to thefollowing program: ramping the temperature at a rate in the range offrom about 0.1° C./min to about 80° C./min from about 35° C. to amaximum temperature in the range of 300° C. to 420° C., and holding atthe maximum temperature for at least 4 hours.
 16. The method of claim 15wherein the ramp rate is about 30° C./minute.
 17. The method of claim 15wherein the maximum temperature is maintained for about 16 hours. 18.The method of claim 12 wherein the calcining comprises a two-stepcalcination program wherein the catalyst is heated to a selected maximumtemperature twice, with gradual cooling of the catalyst between thecalcinations.
 19. The method of claim 12 wherein the Fe²⁺:Fe³⁺ ratio (wt%/wt %) in the range of from about 3% Fe²⁺:97% Fe³⁺ to about 30%Fe²⁺:70% Fe³⁺ and the calcined catalyst has a maghemite:hematite weightratio in the range of about 1%:99% to about 70%:30%.
 20. The method ofclaim 19, wherein the maghemite:hematite weight ratio in the calcinedcatalyst is about 10%:90%.
 21. The method of claim 1 wherein the weightof nitric acid is 2.8 to 4.5 times the weight of iron in the ironnitrate solution.
 22. The method of claim 1 wherein the weight ratio ofcopper to iron in the iron nitrate solution comprising copper is in therange of 0.002 to 0.02.
 23. The method of claim 1 wherein heating theiron nitrate solution comprises heating the iron nitrate solution to atemperature in the range of 60° C. to 80° C. at a rate of temperatureincrease in the range of 1° C./min to 20° C./min.
 24. The method ofclaim 1 wherein the iron nitrate solution is heated to about 70° C. at arate of about 3° C./min.
 25. The method of claim 1 wherein the Fe²⁺:Fe³⁺weight ratio in the resulting nitrate solution after the heating isabout 30%:70%.
 26. The method of claim 1 wherein the Fe²⁺:Fe³⁺ weightratio in the resulting nitrate solution after the heating is about10%:90%.
 27. The method of claim 1 wherein the Fe²⁺:Fe³⁺ weight ratio inthe resulting nitrate solution after the heating is about 3.3%:96.7%.28. The method of claim 1 further comprising reducing the temperature ofthe precipitation agent solution to a temperature in the range of fromabout 25° C. to about 35° C.
 29. The method of claim 1, wherein theprecipitating agent comprises a compound selected from the groupconsisting of NH₄OH, Na₂CO₃, NaOH, K₂CO₃, KOH, (NH₄)₂CO₃, (NH₄)HCO₃,NaHCO₃ and KHCO₃.
 30. The method of claim 1 wherein the chemicalpromoter comprises a potassium compound selected from the groupconsisting of K₂CO₃, KHCO₃, and KOH.
 31. The method of claim 30 whereinthe weight ratio of potassium to iron is between 0.5 K:100 Fe and 1.5K:100 Fe.
 32. A method of manufacturing a catalyst comprising iron,copper and potassium, comprising: mixing together a selected amount ofmetallic iron or an iron-containing compound, to obtain an iron nitratesolution having a Fe²⁺:Fe³⁺ ratio (wt %/wt %) in the range of from about3% Fe²⁺:97% Fe³⁺ to about 30% Fe²⁺:70% Fe³⁺; heating the iron nitratesolution to a temperature in the range of about 20° C. to 80° C.;preparing a precipitating agent solution; reducing the temperatures ofthe iron nitrate solution and the precipitation agent solution torespective temperatures in the range of 25° C. to 35° C., to obtainrespective low temperature solutions; and precipitating a precipitatecomprising Fe²⁺ and Fe³⁺ hydroxides by reacting the low temperaturenitrate solution with the low temperature precipitating agent at atemperature not exceeding 40° C.
 33. The method of claim 32 furthercomprising slurrying the precipitate, adding a chemical promoter, addingcopper nitrate, and spray drying the slurry to form catalyst precursor.34. The method of claim 32 wherein preparing the iron nitrate solutionhaving a Fe²⁺:Fe³⁺ ratio (wt %/wt %) in the range of from about 3%Fe²⁺:97% Fe³⁺ to about 30% Fe²⁺:70% Fe³⁺ comprises preparing a ferrousnitrate solution by adding metallic iron or an iron-containing compoundand a select amount of nitric acid having a specific gravity less than1.035 and a ferric nitrate solution by adding metallic iron or aniron-containing compound and a select amount of nitric acid having aspecific gravity greater than 1.115 and combining them to provide aniron nitrate solution having a specific gravity greater than 1.01 andless than 1.40.
 35. The method of claim 34 wherein the specific gravityof the ferrous nitrate solution corresponds to a concentration of nitricacid in the range of from about 4 wt % to about 10wt % and the specificgravity of the ferric nitrate solution corresponds to a concentration ofnitric acid in the range of from about 22 wt % to about 27 wt %.